1. Technical Field of the Invention
This invention relates generally to the field of modeling the flow features of an object and more specifically to a nonlinear system and method for modeling the aerodynamic performance features of an object in a wind tunnel.
2. Description of the Prior Art
During the design and development of new aircraft, wind tunnels are used to generate performance data of the aircraft, or portions of the aircraft. Given a sufficient amount of performance data, a computer model of the aircraft performance can be constructed from the performance data. The model is then used to further test and refine the design and development of the aircraft.
From conception to completion, a significant amount of time may be invested in wind tunnel testing. Typically many mock-up portions of the aircraft are tested (e.g., wings, nose, tail, fuselage, or the entire aircraft), and each portion may be tested in varying sizes, e.g., 5%, 10%, 20%, 50%, etc. For each test, the aerodynamic effects of a plurality of geometric configurations and independent variables associated with the object being tested are performed. Such independent variables include, for example, flap positions (leading and trailing edge), slat positions, deflection angles, elevator deflections, rudder deflections, stabilizer position, roll, pitch, and yaw positions, angle of attack, and permutations of such geometric configurations, and power settings such as mach number and velocity (collectively referred to as geometric configurations). For each independent variable, a plurality of dependent variables or physical measurements of the object are made using an elaborate "scale" or balance system attached to the object under test. Such dependent variables include, for example, coefficients of lift, drag, side-force, pitching moment, yawing moment, and rolling moment (C.sub.L, C.sub.D, C.sub.Y, C.sub.M, C.sub.N, C.sub.b respectively), lift, and other similar physical measurements.
Given the test performance data, a model is developed through extensive manual data fitting, analysis, and interpolation to derive values between the actual measurements taken. Evaluation of the aircraft design is generally based on a visual inspection of the fitted curves produced. Cross-plots from different wind tunnel runs are combined to assemble the appropriate data to model the aircraft in flight. Most of the data is fit by simple polynomials, limited to 1 or 2 variables taken at a time. If the engineer needs to have curve fits for many variables at once, a linear interpolation of the data is performed. However, a simple linear interpolation may produce errors if there is insufficient data to determine accurate minima and maxima. In addition, this post-test analysis is very labor intensive, specific to each model and test, and requires large amounts of computing power and time.
Modeling of aircraft requires an exceptionally high degree of accuracy, mainly for safety reasons. In order to achieve sufficient accuracy in modeling the aircraft, over-testing is performed so that potentially important data is not lost. Large scale wind tunnel test time is extremely expensive--typically about $6,000 per hour. Thus, any decrease in the amount of wind tunnel test time would result in a significant cost reduction in the design and development of the aircraft, and a decrease in the design cycle time.
Once a model is developed, aircraft developers may use the model to determine the optimal geometric configuration of the aircraft for a particular flight regime. For example, the geometric configuration of a transport aircraft may be optimized for lift so as to reduce required wing area. This optimization problem is generally very nonlinear. Thus, the linear solutions of the prior state of the art are often inadequate. In addition, using the prior art methods of modeling, a model is not readily available during testing. The model is created much later in the design process. Thus, after the post-test analysis is complete, subsequent wind tunnel tests are required to focus on interesting points in the aircraft design. This results in increased expense and design time.
It would be desirable to provide an efficient, accurate, and robust system and method for testing aircraft in wind tunnels by which the number of configurations tested is minimized, the model provides a good representation of "in between" or missing configurations, the post-testing analysis to generate a model is eliminated or significantly simplified, and the generated model reflects inherent nonlinearities. In addition, it would be desirable to provide a model in "real time" so that wind tunnel testing can be modified "on the fly" to focus on interesting points (e.g., maximal lift).